Microbuzzer.com · Posted on 08-08-2010 by microbuzzer

8 ohm speaker impedance frequency

8 ohm speaker arising from the resonance in the 400Hz, its resistance increased from 8 ohms to 10 ohms, at 10KHz, its resistance began to increase, to 1MHz, 100 ohms. In the absence of the audio signal, the final output is a symmetrical square wave, although speaker impedance of 100 ohms, it still consumes power, it will reduce efficiency. Moreover, because high-frequency pulse signal when added directly to the trumpet, will cause more serious RF radiation. When the speaker lead longer, the lead will show distributed capacitance, on the one hand makes the signal sent to the weakened, on the other hand will increase the radiation. These are not promising.

So, usually, or want to add a filter at the output. If the high frequencies, showing capacitive load, then an inductor in series can improve its properties. The series inductance high-frequency components can not flow to the load, thus reducing the radiation, but also improve efficiency. We can simply assume that the inductor should be at 30KHz when the inductance is equal to 8 ohms, which can calculate the inductance value is 42.4 uH.

If the D class amplifier in 250KHz switching frequency, then this frequency, its inductance is 66.7 ohms, the impedance of speakers by about 8 times a large. Therefore, it is the loss in the switching frequency is not very serious. It should be noted that the inductor because the current flowing through the relatively large, so thick the wire should be used. If the 8-ohm on to produce 1.2 watts of power, then the current is equivalent to 387mA (rms), the peak current will reach 550mA. Must ensure that such a large current, the inductor will not be saturated and reduced inductance. If the switching frequency is increased 4 times to 1MHz, then the inductance can be reduced four times, became 10uH. At this time the inductor size can be reduced, but increased frequency since its efficiency will be reduced, the distortion will be increased.

Only a series inductance and high-frequency radiation can not effectively solve the problem. Sometimes it even makes the electromagnetic radiation is more severe. Electromagnetic radiation is more sensitive in portable products. Because it may interfere with the machine to the other RF circuits. In order to reduce the high frequency radiation must also re-use a parallel capacitor to filter the signal directly to radio frequency interference.

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